The physiological factors which regulate parturition and the placental production of hormones in women and in subhuman primates are poorly understood. Experimental studies in appropriate animal models are necessary since extrapolation from other species to primates may be unwarranted. The objectives of this proposal are to clarify the relationships among endocrine and vascular events in the initiation of parturition in primates. Major emphasis will be placed on physiological studies in unanesthetized chronic fetal preparations in pregnant rhesus monkeys and baboons in order to answer the following questions: 1) What is the role of the fetal pituitary-adrenal axis in mediating the onset of labor? 2) Is the rate of placental blood flow important in the production of placental hormones? What is the effect of graded reductions in uteroplacental blood flow on the production rate of estrogens, progesterone, monkey chorionic somatomammotropin (mCS) and prostaglandins by the fetus and placenta? 3) Is the placental clearance of dehydroepiandrosterone sulfate to estradiol directly related to placental perfusion? 4) What is the effect of steroid hormones and carbon monoxide on placental O2 transfer? The relationships among steroid hormones in the maternal compartment, fetal compartment and amniotic fluid will be studied by specific radioimmunoassays. Measurements of blood flow will be made by radioactive microspheres and electromagnetic flowmeter techniques and correlated with continuous measurements of uterine activity.